refactor(picker_ui): address resume feature review feedback (#438)

[gustav-fff]

Follow-up to #438 applying the three review notes from @dmtrKovalenko on top of current main.

Changes

• Single resume_state object (lua/fff/picker_ui/picker_ui.lua) — collapse last_file_picker_state / last_grep_picker_state / last_closed_mode into one resume_state = { files, grep, last_mode } table.
• base_path from config — drop the fuzzy.get_base_path() round-trip in M.close(); read base_path from M.state.config directly, as suggested in the review.
• Recreate on active resume — resume() / resume_find_files() / resume_live_grep() now close the live picker and recreate it from the saved snapshot instead of bailing with a notify, so the user can refresh stale results in one keystroke.

Steps to reproduce / verify

:lua require('fff').find_files()
" type a query, hit <Esc>
:FFFResume
" picker reopens with the previous query, results, cursor restored
" — repeat without closing, the active picker is rebuilt instead of refusing

Confirmed make lint-rust clean and stylua --check clean on touched files. Lua linter was not run locally (lua5.4 missing on the runner), CI will cover it.

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